Unless someone is pressuring your life total, 5-7 spells is enough as you gain the cumulative life on each spell. Starting on 15 life, you can "storm" 6 cards for 56 life and be on 71 without any of those cards actually gaining you life. You can also go off on multiple turns to get there but it's a bit harder though you're still only using the single source as a kill instead of a second copy of tendrils or a YWill/regrowth to reuse the first copy. Having multiples in play is obviously better.